Interpretive Summary: The three most important species of bacteria that digest cellulose fiber in the rumen are known to make different fermentation products that are utilized differently by the dairy animal, but little information is available on what factors determine the outcome of competition among these species. We found that when cellobiose (the major product of cellulose breakdown) was in excess supply, bioreactors inoculated with two species contained both species in similar population densities. However, when cellobiose was steadily supplied at low concentrations, one species always dominated the two-membered cultures. The species that dominated was always the strain that had the highest affinity for cellobiose, as determined by the saturation constant measured in pure cultures. Because cellobiose-- despite being the most abundant sugar in the rumen--is usually present in only low concentrations, it appears that one species may be responsible for rmuch of the cellobiose fermentation in the rumen. This research identifies factors that determines which species of fiber-digesting bacteria become most predominant under defined growth conditions. This information provides new strategies for altering the proportions of digestion products, which in turn affects the composition of milk.

Technical Abstract:
Competition among three predominant ruminal cellulolytic bacteria-- Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD-1, and Ruminococcus albus 7--was studied in substrate-unlimited (batch culture) or substrate-limited (continuous culture) conditions with cellobiose as sole fermentable substrate. Substrate-limited continuous cocultures were performed at 39 deg C under CO2 in a stirred reactor continuously fed a modified Dehority medium supplemented with cellobiose (1-4 g/l). Populations of individual species were determined: by characteristic fermentation endproducts; by signature membrane fatty acids; and by oligonucleotide probes homologous to characteristic sequences of 16S rRNA. The results indicated that FD-1 and S85 co-existed in substrate-unlimited coculture with about equal population size, but FD-1 outcompeted S85 for cellobiose in the substrate-limited cocultures. This outcome of competition nbetween these two strains is due to a classical pure and simple competitio mechanism based on affinity for cellobiose. Although the population size of S85 was much higher (>70%) than that of 7 in substrate-unlimited coculture, S85 was replaced by a population of 7 in the substrate-limited coculture, apparently due to selection in the chemostat of a population of 7 with a higher affinity for cellobiose. This strain also outcompeted FD-1 under cellobiose limitation; the mechanism underlying this outcome is not clear, but did not appear to involve production of inhibitory agent.